1. Field of the Invention
The present invention relates to improving safety of a battery pack used in a portable electronic device and, more particularly, to improving safety against a rapid increase of inside temperature of the battery pack.
2. Description of the Related Art
As the function and performance of portable electronic devices, such as notebook computers, hereinafter referred to as a notebook PC, and mobile phones, have improved, the power consumption of portable electronic devices has also increased. Accordingly, the energy density of portable electronic device batteries has further increased. In order to obtain a voltage suitable for an operation of the portable electronic device, a battery used for a portable electronic device is often in the form of a battery pack having a plurality of battery cells accommodated in a housing, with the battery cells connected to one another in a combination of series and parallel connections.
Batteries using a water-soluble electrolyte, such as a nickel cadmium (Ni-Cd) battery or a nickel hydrogen (NiMH) battery, are often used. In addition, lithium ion (Li-ion) batteries with excellent mass energy density and volume energy density have been recently adopted. In the lithium ion battery, cobalt acid lithium is used for a positive electrode, a carbon material is used for a negative electrode, and an organic electrolyte obtained by dissolving lithium salt in a combustible organic solvent is used as an electrolyte. The organic electrolyte is used because lithium reacts with water. Accordingly, a case or a metal jacket is employed to form a cell that structurally seals the lithium ion battery.
In the water-soluble electrolyte battery, even if cell voltage and temperature increase during electric charging thereby resolving the electrolyte and generating oxygen gas, the oxygen returns to original water due to the oxygen cycle or a catalyst plug. However, an electrolyte of the lithium ion battery cell does not return to an original state once the electrolyte is resolved. Accordingly, if the charging voltage or charging current of a lithium ion battery reaches a critical value or the lithium ion battery is used under abnormal conditions that increase the temperature, the electrolyte is resolved and gas is generated. As a result, the internal pressure increases. For this reason, a gas discharge valve is provided on the lithium ion battery in order to prevent explosion.
Typically a battery pack may be configured as a hard pack in which a battery cell is accommodated in a hard plastic case or as a soft pack in which a battery cell is packed within a heat-shrinkable tube. When a plurality of cells is accommodated, the hard pack is often adopted because it is easy. In a hard pack in which a plurality of lithium ion battery cells are accommodated, a protection circuit is generally provided. The protection circuit monitors a charging current, a charging voltage, the temperature of a cell, and the like and cuts off a charging circuit as required. An intelligent battery system may include a protection circuit for accurately measuring residual battery capacity, a controller, and various kinds of sensors incorporated in the battery pack for measuring and controlling charge and discharge.
In many cases, a battery pack mounted in a notebook PC constitutes a part of a housing of the body of the notebook PC. In addition, in order to increase a space where battery cells are accommodated, a so-called extension-type battery pack in which a part of a housing protrudes from the notebook PC may also be adopted.
When a user uses a notebook PC inside an office, it is common to carry the notebook PC between a conference room and each office without putting the notebook PC in a case or a bag. Accordingly, the notebook PC might be accidentally dropped onto the floor. A strong impact on a battery pack may cause short-circuiting or breakage of an internal circuit of a battery cell. As a result, the temperature of the cell may rise rapidly, and a high-temperature combustible gas might be discharged from a gas discharge valve into the battery pack.
When the battery cell is exposed to high-temperature combustible gas, the inside temperature of the battery pack goes up. As a result, more combustible gas may be emitted from the gas discharge valve. The high-temperature combustible gas that has been emitted may be ignited by an electrical circuit inside the battery pack or may spontaneously combust due to high temperature. Since a plurality of battery cells is densely accommodated in a battery pack, there is also a risk that another battery cell will be heated by the high-temperature combustible gas, causing more combustible gas to be discharged from the heated battery cells, and the temperature inside the housing to increase, and as a result, all battery cells accommodated in the battery pack may combust. In addition, the heat energy of the discharged gas is increased as the battery cells approaches a fully charged state, which increases the risk of combustion.